In the photographic industry, one of the parameters affecting the quality of the film is related to the presence of physical defects on the film which may substantially affect the quality of the use which the user may make of the film.
Within the scope of the present invention, the term defect designates on the one hand any objects or particles which may be present on the surface of the photographic film. An example of this would be any dust such as might result from cutting the film, or atmospheric dust which may be deposited on the film by electrostatic attraction or when the film passes over rollers or suitable transport devices. It may also be a case of particles or pieces of film (or of components of the film) torn off the surface of the film and remaining on the surface of the film, or resulting from cutting the film. Moreover, the term defect also designates structural defects of the surface due, for example, to abrasion, or to what is generally known as "exposure stoppage". In fact, these are optical density defects after development due to local inhomogeneity of the film or to an obstacle interposed during the exposure of the film, between the uniform greying light source and the film.
Since the number and nature of such defects are representative of the quality of the film manufacturing process, it has seemed desirable to develop a method and an apparatus enabling the defects present on such a photographic support to be counted in a simple and reliable manner. It is also important to characterize the defects detected in order to identify any weaknesses in the process used for manufacturing the photographic support in question.
Traditionally, the counting and characterization of the defects on a photographic support were carried out manually by an operator after greying and development of the film examined. Such an approach has the drawback of being tedious, of low reproducibility and imprecise.
According to another known approach, a device of the scanner type is used, utilizing a light beam scanning the surface of the film passing at a constant speed in front of the said scanner. This method, apart from the fact that it operates in analog fashion, has the drawback of not affording sufficient resolution because of the size of the light spot. Typically, the size of such a spot is approximately 1 mm.sup.2. In addition, this does not allow correct use for film samples of small dimensions.